1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to a method supporting synchronous Hybrid Automatic Repeat reQuest (HARQ) transmission of Uplink data.
2. Description of the Related Art
As an example of a wireless communication system, Long Term Evolution (LTE) technology supports both Frequency Division Duplexing (FDD) mode and Time Division Duplexing (TDD) mode.
FIG. 1 illustrates a frame structure of an LTE TDD system. Length of each radio frame 102 is 10 ms and the radio frame 102 is divided into two half-frames 104 of 5 ms, each of the half-frames 104 comprises 8 time slots 106 of 0.5 ms and 3 special fields 108 of 1 ms. The 3 special fields 108 are Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS) respectively, and each subframe is formed by two continuous time slots.
Transmission in a TDD system comprises transmission from the Base Station (BS) to User Equipment (UE), referred to as downlink, and transmission from UE to the BS, referred to as the uplink. Based on the frame structure shown in FIG. 1, uplink and downlink share 10 subframes within every 10 ms, and each subframe is configured either for uplink or for downlink. Subframes configured for uplink are referred to as uplink subframes and those configured for downlink are referred to as downlink subframes.
A TDD system supports seven types of uplink and downlink configurations, as shown in an example of the following Table 1, where D denotes a downlink subframe, U denotes an uplink subframe, and S denotes a special subframe comprising three special fields.
TABLE 1Indexes ofUL/DLConfigura-Switch pointSubframe indextionsperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
An LTE TDD system supports the Hybrid Automatic Repeat reQuest (HARQ) mechanism, and under the basic principle the BS allocates uplink resources for UE; UE sends uplink data to the BS using the uplink resources, and the BS receives the uplink data and sends HARQ indication information to UE, which resends the uplink data according to this indication information. Specifically, the UE carries uplink data in a Physical Uplink Shared Channel (PUSCH), the BS carries a PUSCH scheduling and control information in a Physical Downlink Control Channel (PDCCH), and the BS carries HARQ indication information in a Physical HARQ Indicator Channel (PHICH). The foregoing procedure is based in a preconfigured timing relation to determine timing position of a transmission and timing position of subsequent retransmission of PUSCH, including timing relations from the PDCCH and the PHICH to the PUSCH and such relations from the PUSCH to the PHICH, which are referred to by a joint name as HARQ transmission timing.
In order to increase the user's transmission rate, LTE Advanced (LTE-A) is provided. In LTE-A, the technique of combining several Component Carriers (CC) to obtain wider operation bandwidth is referred to as Carrier Aggregation (CA). For example, a 100 MHz bandwidth can be supported by combining 5 20 MHz component carriers. Each CC is referred to as a cell. The BS can configure a UE to operate in multiple cells, wherein one cell is referred to as a Primary cell (Pcell), while other cells are referred to as Secondary cells (Scell).
In an LTE-A TDD system, it is limited that multiple combined cells adopt the same uplink/downlink configurations, so that HARQ transmission timing relation configured for one cell in LTE can be multiplexed completely without extra standardization operate. HARQ transmission timing relation in a prior system like as LTE and LTE-A will be described below.
First, the timing relation from the PDCCH and the PHICH to the PUSCH will be introduced.
As to the timing relation from the PDCCH to the PUSCH, assuming that the UE receives the PDCCH in downlink subframe n, this PDCCH controls the PUSCH in uplink subframe n+k. Value of k is defined in Table 2 below.
Specifically, as to uplink/downlink configurations 1-6, the number of uplink subframes is smaller than that of downlink subframes, and a unique HARQ transmission timing can be configured, corresponding to Table 2 below. A downlink subframe may not schedule a PUSCH, or only schedule a PUSCH in one uplink subframe. As to uplink/downlink configuration 0, the number of uplink subframes is greater than that of downlink subframes, and the PDCCH in each downlink subframe needs to schedule a PUSCH in two uplink subframes. Therefore, an uplink index (UL index) technique is used in the PDCCH to support scheduling the PUSCH in two uplink subframes. For example, when a UE receives the PDCCH in downlink subframe 0, PUSCH in uplink subframe 4 and/or that in uplink subframe 7 are/is scheduled; when the UE receives the PDCCH in the downlink subframe 1, the PUSCH in the uplink subframe 7 and/or that in uplink subframe 8 are/is scheduled.
TABLE 2TDD UL/DLDownlink subframe index nConfiguration012345678904, 76, 74, 76, 716464244344444454677775
As to the timing relation from the PHICH to the PUSCH, in LTE and LTE-A, PUSCH in each uplink subframe is allocated with a PHICH resource set separately. Assuming that the UE receives the PHICH in downlink subframe n, this PHICH controls the PUSCH in uplink subframe n+k. Value of k is defined as in an example of the following Table 3. Specifically, as to uplink/downlink configurations 1-6, the number of uplink subframes is smaller than that of downlink subframes, and a unique HARQ transmission timing can be configured. In Table 3 below, a downlink subframe may not be configured with a PHICH resource set, or may be configured with a PHICH resource set for only one uplink subframe. As to uplink/downlink configuration 0, the number of uplink subframes is greater than that of downlink subframes, two PHICH resource sets are configured in downlink subframe 0 and 5 respectively. For example, when the UE receives PHICH in downlink subframe 0, PUSCH in uplink subframe 4 and/or uplink subframe 7 may be triggered.
TABLE 3TDD UL/DLDownlink subframe index nConfiguration012345678904, 774, 7716464244344444454677775
Second, the timing relation from PUSCH to PHICH in LTE and LTE-A will be introduced.
As to uplink/downlink configuration 1-6, when the UE receives the PHICH in downlink subframe i, this PHICH indicates an ACK/NACK of the PUSCH in uplink subframe i-k, and value of k is as shown in an example of following Table 4.
As to uplink/downlink configuration 0, when the UE receives the PHICH in the 0th PHICH resource in downlink subframe i, this PHICH controls the PUSCH in uplink subframe i-k. When the UE receives the PHICH in the 1st PHICH resource in downlink subframe 0 or in downlink subframe 5, this PHICH controls PUSCH transmission in uplink subframe i-6.
TABLE 4Uplink/downlinkDownlink subframe index nconfiguration01234567890747414646266366646656664746
Two types of scheduling strategies are defined in LTE-A. The first type is cross-carrier scheduling, and the second type is non cross-carrier scheduling. In cross-carrier scheduling, data transmission in a cell is scheduled by the PDCCH sent by another cell, and in non cross-carrier scheduling, data transmission in a cell is scheduled by the PDCCH sent by the same cell.
When uplink/downlink configurations of multiple cells are the same, cross-carrier scheduling can fully reuse the HARQ transmission timing of non cross-carrier scheduling.
FIG. 2 illustrates conventional cross-carrier scheduling and non cross-carrier scheduling. As shown in FIG. 2, both cell1 and cell2 adopt uplink/downlink configuration 1, the number in the uplink subframe denotes a synchronous HARQ process number, the number in downlink subframe denotes a synchronous HARQ process number of the uplink subframe scheduled by this downlink subframe, the field filled with slashes is a downlink subframe, the field filled with a blank and solid is uplink subframes. As to non cross-carrier scheduling, the UE operates at cell2, the BS sends the PDCCH in the downlink subframe 211 to schedule the PUSCH in uplink subframe 201, and then sends the PHICH in downlink subframe 212, triggers UE to resend the PUSCH of uplink subframe 201 in uplink subframe 202. As to cross-carrier scheduling, the BS may send the PDCCH in downlink subframe 311, and may send the PHICH in downlink subframe 312.
Therefore, regardless of cross-carrier scheduling or non cross-carrier scheduling, the timing relation between the timing position of a transmission of the PUSCH and timing position of subsequent retransmission is constant, which is referred to herein as synchronous HARQ transmission.
When frequency domain distances among multiple cells that implement CA are long enough, these cells may adopt different uplink/downlink configurations without interfering with one another. Thus, in subsequent research of LTE-A, one project aims to research how to support HARQ transmission when uplink/downlink configurations of multiple cells are not identical. However, the prior art provides no solution for this problem.